The Jalopy Journal
Discussion in 'The Hokey Ass Message Board' started by 31Vicky with a hemi, Apr 27, 2014.
Any time brutha...
This is part of the confusion between statics and dynamics.
Every weight has an equivalent force, but weight (mass) and Load (force) aren't the same thing.
Something which occurred to me as I was deciding how my car would look as a gasser, is what about the gasser stance effect. after all, it's not just about sprung weight, but where that weight is placed.
By raising the chassis, traction increases, and without adding mass. Why should it? They hooked up better than lower cars. How come?
The higher higher CG means slower rotation back so a more predictable and less bouncy hookup, and tuned suspension fought tire hop, but mainly I think it's better because you're increasing the leverage the chassis has over the wheels. It can push the wheels harder into the ground because the lever arm from car to wheel has increased.
I thought about this as I was riding my skateboard at lunch. I have dozens of different ones, BTW. Radically different in many cases. It's very high tech nowadays. And I have high ones and low ones.
What happens, just like a car, is that there's a sweet spot. Any higher and stability decreases rapidly. Any lower and you lose leverage against the suspension.
When you get that low, you have to put huge forces into the board to do anything, because you have no leverage against the suspension. A high board can turn without effort, in spite of stiff suspension, because of all the leverage.
The suspension and wheels can all be high tech stuff, but the effect is ruined once you get the adjustments out of the sweet spot.
So if you can't reduce unsprung weight, and if the weight positioning is tuned as well as possible, concentrate on getting things aligned so the suspension leverage will work for you.
(Whatever "work" means, and it'll clearly mean different things for street vs. track.)
Interesting observations about the gasser stance-
Remember form follows function.
So - first thing you need is big sort of sticky tires.
How do you get them under the car ? You raise the ass end right ? Of course.
So now the chassis is raked nose down.
You are running ladder bars I'm the gasser era so where's the instant center? It's right about 2"'under your balls some place under the seat. Oh no that's not going to work , you need the bars under the chassis right,?
How do you do that? Level the car or more. How the hell you going to do that? How about a straight axle ? Lets see if that works. Yep and it looks cool too.
Gasser stance - form follows function
All true, but there's more to it than just that. The high CG means that the inertia of the whole car comes in line faster. The car doesn't need to rise on the suspension, because it's nearly topped out already.
When you step on the gas in a powerful car with street suspension, the whole car has to rise as the suspension wraps up, before it actually starts to go. That all takes time, and any microsecond lost at the very beginning of a race is worth real time at the finish.
I don't think it's the high center of gravity as much as it is the L O N G ladder bars are preloaded. Being pre loaded they don't have to weight for the load to work its way thru. They are ready to go like a loaded spring. They ride like shit like that in race ready trim too.
Technology has come along way and things have changed a bunch.
The gasser wheels up launch is sooooo freaking cool but it really waists power. You want the car to shoot off its suspension and traction not up. Once things changed the cars went faster, got lower, and don't launch like that. CG doesn't go above the driveline and they are quicker.
A bit of a can of worms, I'm afraid. Suffice it to say that the "Operational Definition" of weight is perfectly acceptable in physics, and probably the most useful for our present purpose. The important point, and the distinction you're trying to make, is that weight is not the same thing as mass. Weight is a force resulting from something acting on mass, be that gravity or something else.
Our understanding might be affected by the units we use. When doing physics in Imperial units, the units of mass and force are very closely related, e.g. lb and lbf. I find this a practical hands-on way of working and thus prefer to tackle vehicle dynamics in Imperial units. Still, it can be weird to think that a weight of 1lb of mass could be something other than 1lbf, but it should be clear that it could be.
When doing physics in metric units it is customary to express mass in grammes or kilogrammes and weight in Newtons. The static weight of 1kg at sea level on Earth is 9.8N. The fact that it isn't a simple 1:1 ratio makes it easier to consider circumstances under which 1kg might weigh something other than 9.8N.
It's perhaps a good thing I'm only seeing this thread now. The less said about TANNERGANG's engine mounts the better. The fact that it cannot affect weight distribution can be proven by analysing the bending torque at the point where the mounting attaches to the frame and the forces resulting from that and acting on the axles. That's Landseaandair's point, I think. Ulu is right to point out that it might have an effect on frame elasticity, though.
See this about bumblebee theory.
I explained one of the most important reasons for reducing unsprung mass, but one seldom considered, here:
"Here we see the real reason for wanting to get rid of unsprung mass, rather than the usual reason about inertia over bumps resulting in loss of traction. The more unsprung mass there is, the more weight transfer is locked in and not distributable between front and rear by working with roll stiffness and such. But note that just as important as reducing the amount of unsprung mass is ensuring that the CG of the unsprung mass is as low as one can get it. If one could devise a live axle with the pumpkin hanging below the axle, it might be as good as a lighter axle."
You've read it, 31Vicky; you commented at #4 on that thread.
It depends on what ill-effect you're expecting to result from the surfeit of unsprung mass, or lack of sprung mass, at the rear of the chassis. I've listed one and implied another; moreover a practical one is that the attitude of a very light body is more easily altered by adding passengers and payload. It's easy enough to size rear springs to achieve the resonant frequency or static drop you need to achieve your chosen ride quality, but you'd do so based on an assumption of how many passengers and what sort of load in the trunk, etc. If you have sized springs to be ideal with only yourself on board you'd increase the risk of bottoming with a passenger and/or load; conversely if you assumed a passenger/load the ride might be harsher with only yourself on board. The lighter the sprung mass and/or the softer the springs, the more pronounced the difference will be.
It also affects the attitude at which the vehicle sits: the lighter the sprung mass and/or the softer the springs, the more the car drops when you get in. Do you want it to sit right when parked or when moving?
Practical solutions? Don't be unrealistic about ride-quality expectations. If you can live with 2.0-2.5Hz at the back the problem is a lot easier than if you want to waft about at 1.1Hz. Stiffer springs want damping closer to critical, though, if bone-jarring is to be kept to a minimum.
Move as much as possible as far back as possible. Mount everything that has any degree of optionality of location (battery? spare? fuel tank? brake booster? master cylinder? radiator? coolant reservoir? dry-sump tank? heater? air conditioner?) near or preferably behind the rear axle. Set the engine back as far as practical; consider a "divorced" transmission. Increasing the wheelbase forwards helps here, too, more than you'd think. Then set up the chassis to counteract oversteer: soft rear roll stiffness, low rear roll centre, etc.
Rising-rate springs can help, too. Airbags have this characteristic, and can moreover be set up to self-level. That way you can have a reasonable ride quality while constantly self-adjusting for load variations to maintain that just-right stance.
I can't believe this is still going on!....31Vicky with a hemi look at all the trouble you caused by asking a question......LOL Half my post have been deleted...LOL I am going to leave the rest of this thread to you guys and just come back every now and then for a good laugh.....remember it's all just fun and games anyhow....LOL
Man I get so tired of "what color should I paint this " type of bubble gum and pop corn threads. I needed a good meat and potatoes thread.
Yea it's a bit advanced
Thank you sir for a clear and concise explanation, of a subject that I have a clear picture of in my head yet struggled to convey to print (posts #135,#137,#139) I will sleep better now cheers, Colly.
The ideal of course is for everything to be in a line: crank, trans, driveline, pinion, etc, so it doesn't all wrap up under pressure. Yes, modern cars launch better (and straighter!) Anyhow, my point was that if you can't cut weight, tune where it goes.
Honda decided long ago that the closer all the parts were to the CG, the better handling was, because you weren't fighting rotational inertia as hard. Two vehicles can weigh exactly the same, and yet one can steer slowly, with tire slip, while the other grabs and turns on a dime, because it has lower polar moments in every direction.
This is true of anything that rotates AFAIK.
See, I told Vicky that a continental kit would help traction.
You have to promis not to laugh
But I was thinking of running a chute pack , you know the chute pack / battery box or steel plate holder as plan B
Wheelie bars with lead wheels on them?
Exactly - I did this very thing when building and dialing in my jet sprint boat, Everything possible was mounted in and under the front of the engine between the seats and low down so as to keep the ends of the boat as light as possible to aid quick reaction to steering inputs. (darn I miss that boat). cheers Colly
Of course, sometimes you want a vehicle that doesn't swap ends in an eyeblink.
Then you make the perimeter heavier.
this is part 2
Ned was bringing up another dynamic. "Polar Moment of Inertia". In terms we can understand better, lets call it the "Dumb Bell Effect".
Take a long dumb bell with 100lbs on each end and give it an axle in the center. If it's 2ft long it's easy to get it to spin, get the inertia working for you. If it's shorter, say 8in, it's much harder to get it started due to the lack of leverage. Cars with a high CG have a shorter "polar moment" than those which hug the tarmac. I'm purposely keeping it simple and there's a lot more to it, but I think it's beyond the scope of this topic.
Not so sure it's out of scope if it relates.
In the back of my mind I have some ideas of where and how to add weight as Plan B if you will. Steel plate weighs 12X12x1 @ 40. Lbs ( its actually 40.83), so there will be a difference of where it goes I'm sure, in both fore and aft, from center, and concentration.
The frame is basically 2x6 so adding steel in 12x6 size plates to counteract the unsprung weight would be easy. Not that adding weight is good on a hot rod. In the frame just in front of the rear wheels, I could add 20,40,60,80 pounds to each side and no one would ever see this. Now placement of that weight would be wide from center. I could also spread that over 6" for a higher concentration or over 24" for more of a spread.
My thinking is with it being wide, and below the center of gravity it should work like a tight rope walkers beam, keeping him from flipping. Maybe that's wrong thinking if I understand your post?
There's a few other areas I could ad weight but none as both concentrated and inconspicuous.
Then again adding weight is Plan B
Ok, what color should I paint it ?
Not at all, and the effect is familiar to most hotrodders, skaters, and any kid who's played on a merry-go-round.
Get on the merry-go-round and get it spinning, then lean into the center. It suddenly spins faster because you've re-balanced it. The weight didn't change, but its location did. The momentum didn't change. The total energy is constant, but because your weight is closer to the CG the angular velocity had to increase to maintain momentum.
While skating, if you throw your arms out while turning, you'll turn slower. If you bring them in quickly, you'll turn faster.
Take the heavy old bumpers and brackets off a car from the 50's and what happens? All of a sudden it corners without plowing so much because you've reduced the car's resistance to turning. If you crushed those bumpers to a lump and sat them in the middle of the floorboard, the car would weigh the same, but still turn easier, as the polar moment of inertia is reduced.
FOR OUR PURPOSES, All "polar moment of inertia" means is this:
"Polar" means rotating on an axle or pole. It could be imaginary like the North Pole. It's the center of rotation.
moment = torque
How hard is it rotating = how much "moment"
"Inertia" means that things don't just decide to stop or go.
Something must move them, or if they're moving, stop them.
How hard it must try = how much inertia it's fighting
Spinning things want to keep spinning till you hit the brake, and because mass is moving around a pole (axle) we talk of the polar moment.
This applies in any direction: Pitch, yaw, or roll. If the rockers are heavier, the car will roll slower.
A really fast car isn't just light, it has low polar moments.
Which color makes you drive faster?
I personally think this has turned in to a lot of engineer minded people trying to say a lot of big words.....that 99% of us don't know what it means and I wonder if they do........I personally ain't impressed....get back to what was asked.
Great thread! Good to see various theories explained/debated. Too many cars built without enough thought & understanding of priciples involved.
As for color, anything with lots of clear & flake to add to the unsprung weight! See I learnt something!
Axles & 1/2 the springs will have to be painted a very 'light' color of course...
Good Lord, Tanner, without engineers there would be no cars!
That being said, I've met a lot of engineers who were rather ill equipped to be designers, and it usually was because they'd never really built anything in their entire lives. Not even a model airplane. Certainly not a car. Not even a dog house or a hen house, or even a frikkin bird feeder.
Their stuff usually didn't go together too well.
Anyhow, I don't wan't to baffle people with BS here, but I've been an engineer & CNC machine programmer for almost 40 years. Before that I fixed cars and did welding. I also ran some heavy equipment, ran a prototype shop in a big factory, and supervised a substantial machine and fab shop. I've designed and built a lot of stuff, and the physics applies to cars as well as bird houses. The universe makes no distinction.
What I am not is a professional racer. The only racing I've done is street racing, so there are huge holes in my practical knowledge of what fits in what and who ran what when. 'Course it's a huge subject too.
Also I didn't come here to impress anyone with how smart I am. I came here to learn more than anything, but also for fun. I love to talk about cars & such.
I believe it was Mark Twain who said: "Thinkin' is the hardest work there is."
It is too.
This is why so many folks want to short change the advance planning and just get out the torch later, to make things fit.
Errrr....geezeus you got me rantin' like some net noid now. LOL
Ulu....your one of the ones I HAVE ENJOYED reading ....but a lot of these who haven't built, designed, tried to engineer haven't a clue......I didn't mean to sound harsh...I want you engineers to explain it in Country Boy, Racer terms with out the big words....you know us racers look through a windshield dimly...with shallow thought patterns....cause if we had deep thoughts we'd realize we were crazy for getting on a race track with 40 other like minded fools........man!...I scared I ever did it now......hehehe
Thanks. It takes all kinds to make an interesting world ya know.
I've driven fast cars and motorcycles, up to maybe 140 MPH, but Mischo Erban managed to go 85 MPH.
On a skateboard.
I really thought motorcycle racers had the cajones until I started skating again.
(And no, I've never been over 30 MPH on a skateboard myself...)
Here's how I feel about it Tanner,
If a guy is going to offer his time on a threads it's because he really wants to help because he can most of the time. This subject is pretty advanced to explain the how's and the whys.
That requires a level of vocabulary that is used to describe the concept or theory or phenomena and there may be some words we (you) have not heard before because you (we) dont walk in those circles constantly. Any one of these big words can be googled for a better understanding of the meaning. Or you could ask them directly. It's not fair to say question their understanding because you don't. These guys don't come around to prove themselves, they've already done that. They just don't need to and will not bother.
Sometimes a guy such as yourself will offer sound advice but can't really backit up with much more than "believe me, it works". I believed you that it showed results but I still wanted to know "why". Some didnt believe you and went to great lengths to prove you wrong.
Now that leads right into the other times, other times why guys post.
Those are clear as a bell, not complimentary to their standing and need no explaining.
All they need is ignoring.
Everyone should want to learn, everyone should teach if they can.
31Vicky here's a trick you may (or may not) want to try. Set up the car at your desired ride height. Earlier you alluded to not having purchased springs yet - did I interpret that right? So PART of what you are wanting to do (I believe) is to calculate what spring you need - right so far? So IF you have some resources (and it sounds like you do) you could make up a telltale fixture that would tell you exactly how much LOAD the spring will see. By "see" I mean the actual load and therefore the actual spring force you'd need to design to. Remember my comment about the angle od the spring being something you'd need to consider? That's partly what I am getting at. As an aside - let's say you have a spring mounted at a.....sorry please hold we are off to my sons game......TBC
Well I do have the springs in this case for this one car And they are transverse leaves. But i do not have springs for the next one, there's 2 in ln line right now and looks like #3 wants in line too.
Sound like you are talking about coils or coil overs, is that right?
Earlier race cars and sports cars(XKE) example. were designed before All the new computer programs and such. That is the same place we are with our hot rods. What they did was go to a test track with a multitude of shocks (dampers) and springs and the test driver or engineer tried different combinations that would work the best. All this discussion on this forum is not going to replace that. A tried different springs and shocks on my hot rod to better it.
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